A flow of cosmic rays constantly bombards Earth. Primary cosmic rays consist of single protons (about 90% of all cosmic rays) and alpha particles (majority of the remaining 10%). When these primary cosmic rays hit Earth's atmosphere at around 20,000 m above the surface, the impacts cause nuclear reactions, which produce pions. These pions decay into a muon and muon neutrino at about 9000 m altitude. Many muons decay on the way down into neutrinos and an electron while others reach the surface, and there are still enough particles to be detected fairly easily. About 7,200 muons rain down on each square meter of Earth every minute. This flux is approximately uniform over the Earth's surface.
Muons are electrically charged unstable elementary particles with a mean energy of about 3 GeV, which rain down upon the surface of the earth, traveling at almost the speed of light. The muon has an average half-life of 2.2·10−6 s and weight of 1.88√10−28 kg. The angular distribution of the muons is proportional to cos2α, where α is calculated from the vertical direction.
Cosmic muons observed at sea level come from the decay of unstable pions produced in the upper part of the atmosphere in amounts decreasing with decreasing altitude starting downwards from about 20,000 meters. These pions are produced in nuclear collisions with the air of extraterrestrial protons and a small amount of other nuclei. The muons are not monoenergetic. FIG. 1 shows the most recent and most accurate measurements of the momentum p of the muons, presented by David Gertsle in “Cosmic ray flux study”, Oct. 17, 2007. Here muon energy E is related to the momentum and the muon mass m by Einstein's formula E2=p2+m2.
Various detection techniques were proposed for muons detectors. Muon detectors described below are presented here for the purpose of proof of the systems feasibility. However it does not limit the concept of the present invention to this particular type of detectors.
Cloud chambers with supersaturated vapor and bubble chambers with high pressure liquid were widely used in the past. They allow visualizing the muon trajectory. If the chamber is equipped with a three-dimensional coordinate system, the muon incident angle and coordinate can be measured. Thick layers of photoemulsion were the first detectors used to the muon registration.
The most suitable types of muon detectors for the current system are wire chambers and drift chambers. The wire chambers consist of very large number of parallel wires, where each wire acts as an individual detector. A particle leaves a trace of ions and electrons, the latter drift toward the nearest wire. By marking off the wires which had a pulse of current, one can see the particle's path. Several planes of wires with different orientations are used to determine the position of the particle very accurately. One embodiment of wire chamber detectors is shown in FIG. 2. Typically the chamber I has two windows 2 and 2a. Gas pump 3 is connected with the chamber by inlet and outlet pipes 4 and 5. Three wire gratings are inserted between the windows: two cathode wire planes 6 and 7 and a sense wire plane 8 located in between. Output 9 yields a signal caused by a muon passing through the chamber. Varying voltages applied from the source 10 to the anode wires produce a field in which ionization electrons cause an avalanche towards the nearest sense wire. Additionally the wire detector can be equipped with scintillation detectors. They may be located at the windows 2 and 2a and measure the time of flight for each muon passing the system. The knowledge of the time of flight helps to estimate the muon velocity.
Alternatively drift chambers can be implemented for muon coordinate measurement in the present invention. The coordinate resolution in best muon detectors (such as drift tubes) can be as good as 50 micrometers.
Additionally, a scintillation fiber detector may be used for muon sensing. Such detector has a good spatial resolution. They can be made by forming layers of plastic optical fibers made out of scintillator material coated with a lower refractive index cladding. These can typically have a diameter of 0.5 to 1 mm. The small size of each independent scintillator means that many readout channels (typically tens of thousands) are required, and it is not practical to equip each one with its own photomultiplier. One solution to this is to gather the fibers into a bundle and connect to an image intensifier. This amplifies the light while maintaining an image, which can then be viewed with a CCD camera, and the position on the image associated with a particular fiber.
Since other particles are stimulating the detector as well, a system of two detectors was proposed to avoid false muon detection. Other particles originating from i.e. terrestrial radiation will also cause stimulation, but those particles disappear after passing the short distance, because they are absorbed by nuclear interactions. The detection that occurs almost instant in both detectors is considered as a successful detection of a muon. Muons shielding is not limited to above mentioned additional detector; any other types of shielding can be in order to separate muons from other charged particles.
A sandwich of two coordinate detectors located along the muon path allows simultaneous detecting both the incident angle of the muon and its locations.
It is known that muons easily penetrate most of the materials, because they have only electromagnetic interactions. However an increase of the muon deflection due to Coulomb scattering is observed when they pass materials with high atomic number Z such as nuclear or gamma-ray-shielding materials, Two materials that can be used to make an atomic bomb: plutonium-239 and highly enriched uranium with at least 20 percent of uranium-235. Since both materials have high Z numbers, both can be detected by muon technique. Probability of muon deflection angle forms a Gaussian distribution with a zero mean angle and a width that depends on the material Z number. While muon deflection in 10 cm of aluminum is up to about 10 milliradians, it reaches a value of about 80 milliradians in uranium and plutonium.
Current technologies for nuclear material detection are limited to X-ray and Gamma ray equipment. Both systems must be accurately handled, and their emissions properly controlled. There is a need for reliable and safety system to unveil hidden nuclear materials. Muon detection technique provides a safety alternative with improved penetration ability. The present invention is a continuation-in-part of U.S. patent application Ser. No. 11/626,920, which discloses a system and method for nuclear material detection using muons; this patent application is fully incorporated herein by reference. Since cosmic muons are not monoenergetic, the distribution of the scattering angle depends on the muon energy, the atomic number Z of the material, and the thickness of materials traversed by the muon. There is a need to take into account the distribution of muons' energies when an observation of muon scattering caused by high Z material is performed. A method and system for taking into account the low energy muons contribution into the final measurement result was discussed in details in U.S. patent application Ser. No. 11/947,058 by the same inventors as the present invention; it is fully incorporated herein by reference. There is a need to improve the detection sensitivity and reliability by further optimization of the muon detectors and the system modification for enhanced sensitivity to the high energy muons.